Ok. Well the 10g tank isnt suitable for goldfish. You need about 50g for 3 goldfish. The bigger tank you have would be ok, not ideal, but they would better there than in a 10g. You really need to rehome those goldfish, either find someone who can look after them properly, move them into the bigger tank and return the fish you have in there, or plan to get a bigger tank or pond for them in the future.
With regards to your questions, you are basically asking for a complete guide to fishkeeping in 1 post. There are a bunch of stickies here.
https://www.aquariumadvice.com/forums/showthread.php?t=154837
Articles to Help you Get Started with your Aquarium
Read these, especially the ones about cycling.
Specific brief responses to your questions posed.
Temperature? Depends on the specific fish. Typically for a freshwater tropical tank around 75f will suit most species. Do some research on the fish on google. There are lots of guides out there and the temperature range is always there. Find a temperature that is within the range of all your fish.
How many fish? A rule of thumb is 1" of fully grown fish per gallon of water, but this kind of breaks down when you get large messy fish like goldfish. You also need to consider that some fish need to be kept in groups, so say 6 danios or tetras as a minimum. Make sure you base your fish size on adult size, not what they are currently.
Good fish mixes? Again you need to look more into the specific fish you want, danios, tetras, barbs are typically good community fish. Things like bettas don't mix well with other bettas and other "showy" fish.
How often should i clean? Typically you should be doing enough water changes to keep nitrate below 40ppm. If you have to do more than 50% water change per week to achieve this, you are overstocked. You might want to do filter maintenance every 4 to 6 weeks.
How to clean? Water changes with a syphon, this will also vac your gravel. Wipe clean the glass to remove algae with a suitable algae sponge. Rinse filter media with tank water, dont overclean the filter or you will kill off your beneficial bacteria (see above comments about cycle).
Does water need to sit? No, this used to happen when water was always treated with chlorine, but chloramine is more often used now and this doesn't degas the same. Treat new water with water conditioner, like seachem prime or api tap water conditioner.
I will say again, read about the nitrogen cycle and how to cycle your tank. If you cant test for your water parameters, do 25 to 30% water change daily until you have a test kit and can better judge how often you need to change water while your tanks cycle.
